I was off to the fishing shop to buy a new reel for my 7 weight trout rod.

I was lgoing to buy a Shimano Ultegra, and one spare spool. While I was there, other reels with large arbor, and 3 interchangeable cartridges caught my eye.
Has anyone here had any positive experiences with cartridge reels? There seems to be alot of large arbor trout reels, with 3-4 plastic cartridge spools, with a nice hold-all reel bag on the market in NZ. They are also fairly inexpensive.

This seems to be too good to be true, in these situations there is usually a catch, and you get what you pay for.

I have been using various models and sizes of STH reels (made in Argentina) with the plastic cassette spools (which I'm assuming you are calling cartridges) since 1991 and have not had any problem with any of them. I like them so much that I own 3 #2's (line 5,6,7) 2 #3's (line 8,9.10), and 4 #4's (line 11,12,13). I use the #4's on 2-hand rods with spey lines in the 7/8-9/10 range for steelhead and they have worked flawlessly. One of my #4's (all of them are the DDR #4 model) I've owned since 1994 when they first hit the market and it has landed fresh chum salmon of over 20lbs and chinook salmon of 35lbs.

I've also gotten these STH cassette reels for my wife and my two teen sons.

I've had people tell me they won't hold up because of the plastic cassette spool inserts; but I've not had a problem and the several friends of mine who also use them have not had any problems with them either. The reels I'm referring to have the plastic spool cassettes sandwiched between the cast aluminum side plates of the reel spool, and they run in price from $90.00-$175.00 USD.